The bacterial reaction center (RC) is the membrane protein involved in the initial electron transfer step in photosynthetic bacteria. The subsequent electron transfer reaction from cytochrome c2 (cyt) to the RC is an important step in the photosynthetic electron transfer chain. This project is a study of intermolecular electron transfer between cytochrome c2 and the RC from the photosynthetic bacterium Rhodobacter sphaeroides. A recent advance that motivates this study is the determination of the high-resolution x-ray crystal structure of the cyt:RC complex (H. Axelrod et. al. (2002) J. Mol. Biol. 319, 501-515) that raises important questions about role of the cyt:RC complex in binding and electron transfer reactions. Three features of the reaction that will be investigated are; 1) the structure and energetics of the protein-protein association involved in the binding of the two proteins, 2) the inter protein electron transfer of the cytochrome bound on the RC surface and 3) the dynamics of protein docking into position for electron transfer. The molecular basis for these processes will be examined using a variety of techniques including site directed mutagenesis of the RC and cytochrome c2, structural determination of cyt:RC complexes, laser kinetic measurements of first order and second order electron transfer rates and computational modeling. [unreadable] [unreadable] The specific aims of this project are to: 1) Modify specific residues on the interaction surface between RC and cyt by site directed mutagenesis. The residues to be targeted include residues involved in hydrogen bonding interactions between the two proteins and a surface motif of three Thr residues on the front surface of the cyt that appears to be important for binding. The effects of these modifications on the binding constant for the complex, rate of electron transfer from the bound cyt and the rate of association of the complex will be studied. 2) The effect of osmotic pressure on binding and electron transfer will be investigated to determine the role of bound water. 3) The driving force dependence of the rate of electron transfer will be studied to characterize the electron transfer mechanism. 4) The reaction between horse heart cyt c and the RC, which is believed to have a different binding configuration, will be studied. 5) X-ray crystal structures of different cyt:RC complexes will be determined. 6) Computational approaches will be used to model the binding and dynamics of the cyt:RC complex. [unreadable] [unreadable]